Unexpected results when using first day of the week based on locale

Hi, I've created a sample project configured with First day of week set to Default based on locale in project settings. Then I have this demo_user account setup with Language is Dutch, Netherlands.

I have the following function created in a microflow: parseInteger(formatDateTime($Test/EndDate,'w')). When $Test/EndDate is filled with 09-07-2017 00:00, I get the week nr 28. I would expect this to be 27, because of the locale language settings.

When changing the First day of week to Monday, I get the expected result 27.

What did I miss?